Thursday, June 19, 2008

Polar bears occur only in the Northern Hemisphere. Their range is limited to areas in which the sea is ice covered for much of the year. Over most of their range, polar bears remain on the sea-ice year-round or visit land only for short periods. Polar bears are common in the Chukchi and Beaufort Seas north of Alaska. They occur throughout the East Siberian, Laptev, and Kara Seas of Russia and the Barent's Sea of northern Europe. They are found in the northern part of the Greenland Sea, and are common in Baffin Bay, which separates Canada and Greenland, as well as through most of the Canadian Arctic Archipelago. Because their principal habitat is the sea-ice surface rather than adjacent land masses, they are classified as marine mammals. In most areas, pregnant females come ashore to create a den in which to give birth to young. Even then, however, they are quick to return to the sea ice as soon as cubs are able. In some areas, notably the Beaufort and Chukchi Seas of the polar basin, many females den and give birth to their young on drifting pack ice.

Polar bears are most abundant in shallow-water areas near shore and in other areas where currents and upwellings increase productivity and keep the ice cover from becoming too solidified in winter.Despite apparent preferences for the more productive waters near shorelines and polynyas (areas of persistent open water), polar bears occur throughout the polar basin including latitudes >88.

Because they derive their sustenance from the sea, the distribution of polar bears in most areas changes with the seasonal extent of sea-ice cover. In winter, for example, sea-ice extends as much as 400 km south of the Bering Strait, which separates Asia from North America, and polar bears extend their range to the southernmost extreme of the ice . Sea-ice disappears from most of the Bering and Chukchi Seas in summer, and polar bears occupying these areas may migrate as much as 1000 km to stay with the southern edge of the pack ice.Throughout the polar basin, polar bears spend their summers concentrated along the edge of the persistent pack ice. Significant northerly and southerly movements appear to be dependent on seasonal melting and refreezing of ice near shore .In other areas, for example, Hudson Bay, James Bay, and portions of the Canadian High Arctic, when the sea-ice melts, polar bears are forced onto land for up to several months while they wait for winter and new ice.

Until the 1960s, the prevalent belief was that polar bears wandered throughout the Arctic. Some naturalists felt that individual polar bears were carried passively with the predominant currents of the polar basin.Researchers have known for some time that is not the case. However the advent of radiotelemetry, including the use of satellites,detailed knowledge of polar bear movements was not available.

•Leads are water channels or cracks through ice which may remain open (ice free) for only a few minutes to several months, depending upon weather conditions and water currents. Polar bears hunt seals in the leads, using sea ice as a platform.

•Polynyas are areas of water, surrounded by ice, that remain open throughout the year due to winds, upwellings, and tidal currents. Polynyas are important breathing and feeding areas for wintering or migrating marine mammals and birds.

3. Some polar bears follow the lower edge of the ice pack year-round, making extensive migrations as the ice recedes and advances.

4. Some polar bears spend part of the year on land. They have been found as far inland as 200 km (124 mi.).

•Polar bears in warmer climates may become stranded on land. In summer, sea ice melts along the coastlines, and pack ice (floating sea ice, or floes, not connected to land) moves north.

•Most pregnant females spend the autumn and winter on land in maternity dens.

5. Air temperatures in the Arctic average -34°C (-29°F) in winter and 0°C (32°F) in summer. The coldest area in winter is northeastern Siberia, where the temperature has been recorded as low as -69°C (-92°F). The warmest areas in summer are inland regions of Siberia, Alaska, and Canada where temperatures can reach as high as 32°C (90°F).

6. The ocean temperatures in the Arctic are about -1.5°C (29°F) in summer. In winter the ocean temperatures can drop to -2°C (28°F), at which point seawater begins to freeze.

Migration

1. Polar bears travel throughout the year within individual home ranges.

a. Home range size varies among individuals depending upon access to food, mates, and dens.

b. Home ranges tend to be larger than for other mammal species because sea ice habitat changes from season to season and year to year.

(1) A small home range may be 50,000 to 60,000 square km (19,305/23,166 square m.). Small home ranges can be found near Canadian Arctic islands.

(2)A large home range may be in excess of 350,000 square km (135,135 square mi.). Large home ranges can be found in the Bering or Chukchi Seas.

c. Polar bears don't mark or defend their home ranges.

2. Polar bears show "seasonal fidelity": they remain in the same area during the same season.

3. Polar bears are capable of traveling 30 km (19 mi.) or more per day for several days.One polar bear was tracked traveling 80 km (50 mi.) in 24 hours. Another polar bear traveled 1,119 km (695 mi.) in one year.

Population

1. The world polar bear population is estimated to be between 21,000 and 28,000 individuals.

2. Due to governmental regulations on hunting, the population has increased from an estimated 10,000 polar bears in 1968.